The present invention is an improvement on the invention described and claimed in U.S. Pat. No. 3,590,344, granted June 29, 1971, to the same applicant and the saame assignee as the present invention. Specifically, the present invention provides an improved method of making the light activated semiconductor controlled rectifier described and claimed in U.S. Pat. No. 3,590,344.
Semiconductor controlled rectifiers (SCR), sometimes called "thyristors", are non-linear, solid state devices that are bistable. That is, they have both high and low impedance states. They commonly have four-layer, PNPN structures. Semiconductor controlled rectifiers are usually switched from the high impedance state to the low impedance state by means of a control or gating signal applied to the cathode-base region of the device. Semiconductor controlled rectifiers can also be gated by light radiation incident on the base regions.
Light activated semiconductor controlled rectifiers are well-known for their efficient switching capabilities. The incident light generates electron-hole pairs in the vicinity of the reverse bias center PN junction which instead of recombining, are swept across the junction and increase the anode to cathode current. This current increases with increased light, increasing the current gain of the PNP and NPN transistor equivalents of the structure. If the photocurrent is high enough, it will switch the silicon controlled rectifier from the high impedance blocking state, to the low impedance conducting state.
A major problem with such light activated silicon controlled rectifiers is rapid generation of sufficient photocurrent to gate the device. A standard four-layer semiconductor controlled rectifier can have its base regions directly light irradiated only at the edges along the periphery of the semiconductor body. Such "edge-fired" devices, however, have a very small area sensitive to the incident light and in turn have a relatively long switching time or current rise time on turn-on, i.e. the time required to switch from the blocking mode to conducting mode. Further, encapsulation of such "edge-fired" devices is difficult.
The sensitive area has been greatly increased by irradiating the base regions through the cathode emitter. That is, light with wavelengths in the very near infrared penetrate through the cathode-emitter region to generate electron-hole pairs deep in the interior regions of the device. Moreover, the light is preferably of a wavelength which partially passes entirely through the four conductive regions of the semiconductor body, and reflects from reflective means disposed in the opposite major surface of the body. The light energy is thus reflected back into the semiconductor body and is further absorbed to generate increased electron-hole pairs through a larger volume of the body, see U.S. Pat. No. 3,590,344. The wide distribution of the light through the body causes a larger area of the device to be initially turned-on, increasing the dI/dt capability and decreasing the turn-on time of the SCR.
A fundamental problem with such light activated semiconductor controlled rectifiers has been the making of the light radiation reflective means. A simple method of etching the reflective means into the semiconductor body is described in the parent applications. However, that method was not fully satisfactory. Deterioration of the reflectivity of the etched surface occurred in the subsequent alloying step. In addition, alloy would occasionally funnel along the etched dislocations during the alloying step and short-out the reverse blocking junction of the semiconductor controlled rectifier.
The present invention overcomes these difficulties and disadvantages. It provides a method of making the light activated semiconductor controlled rectifier described in U.S. Pat. No. 3,590,344 without reducing the quantative yield of the device.